A transmission method of a discrete multi-tone (DMT) modulation scheme is used for a metallic line of an access system such as VDSL (Very high bit rate Digital Subscriber Line). Recently, applying the DMT modulation scheme to an optical transmission technique is researched and developed in accordance with the increase of traffic on a network.
The DMT modulation scheme is a multicarrier transmission technique based on an orthogonal frequency division multiplexing (OFMD) technique. In the DMT modulation scheme, data are allocated to a plurality of subcarriers (SC) having different frequencies, respectively, the allocated data is modulated based on a multilevel degree and a signal power in accordance with a transmission characteristic for each SC, and a result of the modulation is transmitted as a DMT signal. As a multiple-value modulation scheme used for data of each SC, Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) are cited.
However, a dip occurs in a frequency characteristic of the DMT signal due to a chromatic dispersion on a transmission channel or due to interaction of the chromatic dispersion and a frequency chirp of a transmission device. When a specific frequency component of the DMT signal disappears by the dip (hereinafter referred to as “fading”), a signal-to-noise ratio (SNR) of a reception side deteriorates, and hence a transmission capacity and a transmission distance of the DMT signal reduce.
In response, an inventor tried to remove one of sidebands of an optical spectrum waveform of the DMT signal so as to reduce an influence of the fading (see e.g. Japanese Laid-open Patent Publication No. 11-205240), and to compensate for the nonlinearity of a response of an optical device such as an optical transmission device with the use of a digital filter (see e.g. U.S. Patent Publication No. 2013/0272719).